1. Field of the Invention
This invention relates to a rotational speed control device during idling of an engine, and more particularly to improvements in an idling rotational speed control device suitable for use in a motor car, comprising: intake air flow rate control means for controlling intake air flow rate during idling; a step motor for driving the intake air flow rate control means; and an electronic control circuit for controlling the step motor in accordance with the operating conditions of the engine.
2. Description of the Prior Art
In general, in engines including engines for motor cars, etc., there have been used idling rotational speed control devices for holding smooth rotating condition of the engines even when a driver removes his foot from an accelerator pedal and a throttle valve is fully closed, i.e., under no load condition, etc. One of the idling rotational speed control devices of the type described includes: intake air flow rate control means for controlling intake air flow rate during idling, i.e., when the throttle valve is fully closed; a step motor for driving the intake air flow rate control means; and an electronic control circuit for controlling the step motor in accordance with the operating conditions of the engine. For instance, in case this idling rotational speed control device is used in an engine comprising: a throttle body having a throttle valve for controlling intake air flow rate; and a fuel injection valve for injecting fuel to air taken into an intake manifold through the throttle body, a bypass passage for bypassing the throttle valve of the throttle body and, during idling, i.e., when the throttle valve is fully closed, the opening area of the bypass passage is controlled by an air flow rate control valve driven by the step motor so as to control the intake air flow rate during idling. With the idling rotational speed control device provided with the air flow rate valve driven by the step motor as described above, in the electronic control circuit, the increase or decrease in pulse number for driving the step motor from the reference position is calculated, whereby the opening degree of the air flow rate control valve is brought into register with the position of the step motor stored in the electronic control circuit, so that the opening degree of the air flow rate control valve can be detected. However, during operating conditions of the engine, the opening degree of the air flow rate control valve should not necessarily correpond to the reference position, and, if the opening degree of the air flow rate control valve is forced to be one corresponding to the reference position during normal running condition, then there occurs a possibility of that the operating performance of the vehicle is impaired. Consequently, normally, at the time of starting the engine, firstly the step motor is driven in either fully opened direction or fully closed direction, the reference position is passed therethrough once, the number of the steps in the step motor is brought to correspond to the opening degree of the intake air flow rate control valve, and thereafter, the normal control is effected. However, in the abovedescribed method wherein the reference position is passed therethrough in the beginning step of control at the time of starting the engine, it may take a time to pass through the reference position depending on the correlation between the reference position thus set and the opening degree of the air flow rate control valve when the engine is stopped in operation, and, during this time taken, the idling rotational speed is not effected, thus presenting a disadvantage particularly when rapid control action is required as at the time of starting the engine.
In addition, there have been proposed the provision of a potentiometer for constantly detecting the opening degree of the air flow rate control valve or the provision of a position sensor such as a limit switch for detecting the specific opening degree of the air flow rate control valve. Each of the abovedescribed provisions results in complicated arrangement and decreased reliability, and moreover, such problems are presented as decreased accuracy in the detecting performance of the position sensor, i.e., dispersion in the accuracy in the detecting performance thereof depending on the positions where the position sensor is installed.